Study objectives We examined the impact of adding a single high-melanopic-illuminance task lamp in an otherwise low-melanopic-illuminance environment on alertness, neurobehavioral performance, learning and mood during an 8-h simulated workday. Methods Sixteen healthy young adults [mean(±SD) age = 24.2±2.9, 8F] participated in a 3-day inpatient study with two 8-h simulated workdays and were randomized to either ambient fluorescent room light (~30 melanopic EDI lux, 50 lux), or room light supplemented with a light emitting diode task lamp (~250 melanopic EDI lux, 210 lux) in a cross-over design. Alertness, mood and cognitive performance were assessed throughout the light exposure and compared between conditions using linear mixed models. Results The primary outcome measure of percentage correct responses on the addition task was significantly improved relative to baseline in the supplemented condition (3.15±1.18%), compared to the ambient conditions (0.93±1.1%; FDR-adj q=0.005). Additionally, reaction time and attentional failures on the psychomotor vigilance tasks were significantly improved with exposure to supplemented compared to ambient lighting (all, FDR-adj q≤0.030). Furthermore, subjective measures of sleepiness, alertness, happiness, health, mood and motivation were also significantly better in the supplemented, compared to ambient condition (all, FDR-adj q≤0.036). There was no difference in mood disturbance, affect, declarative memory, or motor learning between the conditions (all, FDR-adj q≥0.308). Conclusions Our results show that supplementing ambient lighting with a high-melanopic-illuminance task lamp can improve daytime alertness and cognition. Therefore, high-melanopic-illuminance task lighting may be effective when incorporated into existing suboptimal lighting environments.